1. Field of the Invention
The present invention relates generally to magnetic sensors, such as magnetoresistive spin valve (SV) sensor structures for magnetic heads of hard disk drives, and magnetic tunnel junction (MTJ) structures for use as memory cells and as magnetoresistive sensors for magnetic heads of hard disk drives, and more particularly to the use of low cost, high melting point alloy materials for antiferromagnetic coupling (AFC) films in laminated ferromagnetic layer structures of the SV and MTJ devices.
2. Description of the Prior Art
One type of a GMR read head is a sandwich structure comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metallic electrically conducting spacer layer, typically copper (Cu), in which the magnetization direction (magnetic moment) of one of the ferromagnetic layers is fixed or pinned, while the magnetization direction of the free or sensing ferromagnetic layer is free to rotate. This type of GMR device is referred to as a spin valve (SV) magnetoresistive sensor in which only the free or sensing ferromagnetic layer is free to rotate in the presence of an external magnetic field in the range of interest for the sensor. A basic magnetoresistive SV sensor is described in U.S. Pat. No. 5,206,590.
A magnetic tunnel junction (MTJ) device has two ferromagnetic layers separated by a nonmagnetic electrically insulating layer, called the tunnel barrier layer, which is typically formed of alumina. The magnetization direction of one of the ferromagnetic layers is pinned while the magnetization direction of the other ferromagnetic layer is free to rotate relative to the pinned layer. A tunneling current flows perpendicularly through the tunnel barrier layer depending upon the relative magnetization directions of the two ferromagnetic layers. Such MTJ devices are used as memory cells and as magnetoresistive sensors in magnetic heads. A basic MTJ sensor is described in U.S. Pat. No. 5,841,692.
The SV and MTJ devices have been improved by substitution of one or both of the free and pinned ferromagnetic layers with a laminated layer structure comprising two ferromagnetic films that are antiferromagnetically coupled to one another in an antiparallel orientation by an antiferromagnetically coupling (AFC) film. These improved devices are described in U.S. Pat. Nos. 6,153,320, 5,408,377 and 5,465,185. The sign and strength of the interlayer coupling for the thin AFC films is important for the performance of these laminated structures, and it depends in part upon the coupling film material. Ru as a coupling film material displays very strong antiferromagnetic coupling between cobalt (Co), cobalt-iron (CoFe) and nickel-iron (NiFe) ferromagnetic films even when just ˜3 Å thick. This makes Ru very useful to form pairs of antiparallel oriented ferromagnetic films. It is also useful for many applications that the thickness of the AFC film be as thin as is practicable, because this takes up less space, and because the AFC film must not significantly increase the conductance of the laminated layer structure. Thus, Ru is particularly useful because it is very effective as a thin film coupling layer, and it has also been shown that the strength of its interlayer coupling decreases with increasing AFC film thickness. Finally, Ru is especially useful because it exhibits strong antiferromagnetic coupling for a wide range of ferromagnetic materials.
With particular regard to the present invention, it has been found that current SV designs and MTJ designs with an Ru AFC film can become thermally unstable after prolonged operation at elevated temperatures and/or high bias currents, and this instability manifests itself as a decrease in signal amplitude after prolonged operation under such conditions. In essence, these high temperature operating conditions, in excess of approximately 250° C., constitute an in-situ anneal of the laminated structure which can alter key operating parameters such as the antiferromagnetic coupling field between the anti-parallel ferromagnetic layers of the pinned layer structure. It is the object of the present invention to improve the stability of the sensor by employing materials less susceptible to the annealing effects attending such high temperature operating conditions, and to reduce the cost of fabrication of SV and MTJ devices by emphasizing materials less costly than Ru, Os, Ir and Rh, which have been used in the prior art due to their high AFC coupling coefficients. This is accomplished through alloying these materials with other materials of lower cost such as W, Ta, Mo, Nb and Re. For suitable compositional ranges, the cost of targets used to sputter these materials in the fabrication process can be significantly reduced without reducing the AFC coupling coefficient below a level where it would be too low to provide useful sensor characteristics.